French Pat. No. 2,434,377 discloses such a contact link which is mounted on the cyclic-pitch and yaw control chains of the flight controls of helicopters of medium tonnage.
Such a known link comprises two essential elements movable relative to one another, of which one is a link body and the other a link rod which is partially engaged and mounted slideably in the link body, and of which the ends forming the opposite ends of the link must be connected, by means of ball-joint sockets or forks, to the pilot's cockpit on the one hand and to a fixed point on the aerodyne on the other hand, while the electrical microcontactor serves for opening or closing the electrical circuit of the automatic pilot.
The internal arrangement of this link is such that it does not allow any relative shift between the body and the link rod, as long as the operating force exerted on the opposite ends of the link has not reached and exceeded a predetermined value, namely, the dynamometric operating threshold. This contact link thus maintains a zero play between the link rod and the link body, as long as the axial stress on the link is below the predetermined threshold value, and then the link allows a stroke of controlled amount of the link rod relative to the body so as to actuate the electrical contactor only after the threshold has been exceeded.
In this known contact link, the link body has substantially the form of a cylindrical sleeve possessing a symmetrical cylindrical axial recess with two levels, a central part of which forms a bearing of larger diameter connected by means of radial shoulders to two lateral parts forming bearings of smaller diameter. Seated in the central bearing is a ring fixed to the link rod and surrounded by another concentric peripheral ring to eliminate any radial play, while two other rings are arranged on either side of the ring fixed to the rod, and are fitted axially onto the latter during assembly, in order to form movable stops limiting the axial shifts of the rod in the body by coming in contact with stationary stops of the body which are formed by the inner radial shoulders connecting the central bearing of larger diameter to the two lateral bearings of smaller diameter in the body. Two identically prestressed helical springs bear against other inner stops of the body and exert their opposing effects on either side of the axial stack of rings fixed relative to the link rod in terms of translational movement, which is accommodated in the central part of larger diameter of the inner recess of the body, with an axial play corresponding on one side to the stroke allowed under compression and on the other side to the stroke allowed under tension. The two opposing prestressed springs, both working under compression, thus form a detector for detecting the exceeding of the dynamometric operating threshold which corresponds to the prestress load of each of the springs, and such an assembly prevents any axial shift of the rod relative to the link body whenever a tensile force or a compressive force of a value below the prestress value of the springs is exerted on the link, the link rod on the contrary shifting over the low stroke allowed under tension or compression from its position of equilibrium defined by the springs, after the load to which the link is subjected under tension or compression has exceeded the prestress threshold of the springs.
The electrical contactor, usually a quick-break microcontact, is accommodated in the body and fixed to the latter, and this contactor is actuated by the link rod by means of a device for the mechanical amplification of the stroke of the rod relative to the body. This mechanical amplifier is indispensable because the predetermined actuating stroke which the movable contact member of the electrical contactor has to be given in order to change over the latter, starting from the movement of the rod in relation to the link body, is far higher (in a ratio higher than 5) than the very low stroke which the rod must execute in the link body both under compression and under tension. Of course, at the same time, the mechanical amplifier of axial stroke or shift of the rod relative to the link body must amplify this stroke in the two directions of functioning under tension and under compression, must also have a play and a hysteresis which are virtually zero, and must be insensitive to the radial plays between the rod and the link body and to the temperature variations over a wide range extending, for example, from -80.degree. C. to +100.degree. C.
To meet these various requirements, the link described in the abovementioned French patent is equipped with a mechanical stroke amplifier comprising at least one amplifying pincer or lever, but preferably two opposite pincers or levers, each of which is arranged substantially axially in the body, in the position offset laterally relative to the axis of the contact link, and pivots about a pivot pin fixed to the link body. On one side of its pivot, each pincer or lever, on the same side of the end of the rod engaged in the body, has an arm, in the free end of which a notch with two contrary slopes has been made opposite a groove likewise with two contrary slopes and formed round the end of the rod inside the body. Furthermore, a ball is gripped between, on the one hand, the two contrary slopes of the groove of the rod and, on the other hand, the two contrary slopes of the notch of the end of the swan neck-shaped pincer or lever which, on the other side of its pivot on the body, is extended by a second arm coupled to the movable contact member of the electrical contactor in order to actuate it. If the mechanical amplifier comprises only a single lever or a single pincer, the electrical contactor is fastened directly to the link body, whereas if the mechanical amplifier comprises two opposing pincers or levers, the electrical contactor is fastened to the second arm of the lever which does not actuate the movable contact member of the contactor. In both embodiments, any axial shift of the rod relative to the link body under tension or compression is converted into a radial shift of the ball or balls which push back the lever or levers, the latter always pivoting in the same direction for each lever, but in opposite directions of one lever relative to the other when the amplifier comprises two symmetrically opposed levers, $o that the short axial stroke of the rod in the body is thus converted into a radial stroke of much higher amplitude which is transmitted to the movable contact member of the contactor and which is sufficient for actuating the latter. To avoid the effect of the production tolerances of the components, particularly of the amplifier, which are associated with the link body, and to adjust the operation of the stroke amplifier in an appropriate way, means are also provided to allow the radial adjustment of the position of the pivots of the lever or levers of this amplifier. This is obtained by means of a movable plate which supports the pivots and which is fixed to the body by means of screws or equivalent threaded members passing through slots suitably oriented and formed in the supporting plate of the pivots.
Although such a contact link meets the various requirements set out above, it nevertheless has serious disadvantages because it comprises a large number of components without any exact adjustment both in terms of the spring detector for detecting the exceeding of the dynamometric operating threshold and in terms of the mechanical stroke amplifier; it cannot be obtained in a simple way; and its reliability is questionable since it comprises a large number of components which experience considerable wear in service. As a result of this, the production and maintenance costs of such a contact link are high.
With regard to the state of the art, mention should also be made of U.S. Pat. No. 3,902,379, which describes a quick-uncoupling link mounted in the rod assembly of a double flight control between a main cockpit occupied by a trainee pilot and an auxiliary cockpit occupied by an instructor, the quick-uncoupling link being intended to enable the instructor to obtain the remote activation of the flight controls from the auxiliary cockpit and uncoupling between the main flight controls operated by the trainee pilot and the control surfaces and other controlled members of the aircraft in an emergency. The link comprises a cylindrical body, in which a rod is engaged and immobilized in the initial position by means of a ball-type locking mechanism, the balls of which are retained in a cage fixed to the end of the rod and project into inner recesses of the cylinder under the thrust of a central cam on the end of a pin mounted axially slideably in the rod, the cam being retained in the locking position by means of a spring. An electromagnetic, pneumatic or hydraulic actuator controlled by the instructor makes it possible to shift radially a wedge, of which the surface inclined relative to the axis of the link is in contact with the conical surface of a stop carried by the pin sliding in the rod, at the end opposite that having the stop.
The radial shifting of the wedge by the actuator causes an axial shift of the pin carrying the stop counter to the spring, the effect of which is to bring about a radial shift of the balls up to a release position, allowing the link rod to slide freely in the link body. As a result of this, the control movements exerted on the link rod by the trainee pilot are no longer transmitted to the rest of the flight control chain and have no effect on the steered members.
Such a quick-uncoupling link is therefore not the type of contact link forming the subject of the invention, because it has no stroke amplifier and, above all, it is not the relative axial movements of a rod and of a body which allow the actuation of an electrical contactor, but the operation of such a contactor in order to control the functioning of an electromagnetic, hydraulic or pneumatic actuator which releases a locking device initially blocking the link rod and link body in a predetermined axial position, so as to allow them to slide freely relative to one another.
Furthermore, the French Pat. No. 2,570,672 discloses a double automatic tab and power control system for equipping aircraft, which, in a flight control chain between a pilot control and a control surface, comprises an automatically functioning force sensor/comparator with a single throw-over switch with integral mid-point, this device being intended to avoid the need for locating the point of equilibrium of a control surface. This device comprises essentially a cylinder connected at one end to a pilot control and closed at its other end by means of a cover, and a double-acting piston mounted axially slideably in this cylinder, in which the piston is subjected to the opposing actions of two identical prestressed compression springs. The axial stroke of the piston is limited by two stops formed by metal washers connected to electrical conductors feeding a micromotor for controlling the tab of the control surface. The piston is fixed to a force transmission rod which passes radially through two diametrically opposed axial slots made in the cylinder and which is fixed to two branches of a control fork connected to the control surface, these two branches being mounted axially slideably in flats of the cover, but being fixed in terms of rotation relative to the latter. When the axial load to which this device is subjected both under compression and under tension exceeds the prestress threshold of the springs, the piston is shifted from its position of equilibrium until it comes in contact with one of the two stops and thus makes an electrical contact allowing the feeding of the tab control micromotor, so as to cause the control surface to pivot in the direction returning it to its aerodynamic position of equilibrium.
This device therefore has some similarity to the contact link described in French Pat. No. 2,434,377, inasmuch as it is certainly the relative axial shift of a rod and of a body beyond a predetermined force threshold which controls the making of an electrical contact. However, this device does not ensure any stroke amplification, and because of this it cannot be used satisfactorily for the purposes under consideration.
The same is true of the contactor-type safety device described in French Pat. No. 1,131,880 and in its second Patent of Addition No. 70,535, which relates to a composite system of manual and automatic control for aircraft.
This composite control system is a hydroelectric or hydromechanical system capable of ensuring the control of a hydraulic control-surface actuation servomotor according to one or the other of two operating modes, one of which ensures control by hydroelectric means in response to an electrical control signal generated either as a result of the operation of a manual control member or by an automatic pilot, while the other ensures control by hydromechanical means solely in response to the manual control member. In such a system, the contactor-type safety device is at the same time designed as an elastically yielding coupling produced so as normally to ensure transmission without flexibility, whatever the forces necessary for obtaining the shift of the manual control member corresponding to the shift of the control surface, but yield elastically when exceptional forces are exerted on this manual control member, for example, when a sufficient force is exerted on it by the pilot, and switching members are provided in order to be actuated when this coupling yields elastically by an amount exceeding a predetermined value. The contactor coupling of this system comprises two elements, one of which is connected to the output of the control-surface actuation servomotor, while the other is connected to the manual control lever. The rod assemblies for connection to the two elements in the region of the contactor coupling are substantially coaxial, and the arrangement is such that the axial shift of one of the elements ensures the axial shift of the other element, despite a spring subjected to a preload and arranged between two stops set apart from one another and located in an annular receptacle of one of the elements, being capable of moving freely in the axial direction on an extension of the other element. The contacts of the contactor are then opened if, between the two basic elements of the device, there is a relative axial shift which exceeds a predetermined amount, i.e., if the spring experiences the effect of an overload. As an example, one of the elements can carry a pair of pistons, so that the spring stresses these pistons towards fixed positions on the element by means of which each of them is shifted elastically, when the force transmitted to it from the corresponding stop of a pair of stops provided on the other element exceeds the thrust force of the spring. In the system which is the subject of French Pat. No. 1,131,880, a single contactor coupling is used and is mounted between a link connected by means of a mechanical differential to the control surface or to the output of the servomotor actuating this control surface and, on the other side, to a pivoting lever, itself actuated by the manual control lever. In the second Patent of Addition No. 70,535 to the above-mentioned patent, the system comprises two contactor couplings which bend elastically at different force thresholds and of which one is interposed in the mechanical connection between the manual control member and an artificial feeler device, while the other is interposed in the linkage between the artificial feeler device and the control-surface actuation servomotor or the said control surface. The opening of the contacts of these contactor couplings opens an electrical circuit which is designed to cause a reversal of the system, in order to change it to a hydromechanical working mode from the hydroelectric working mode. However, in these various embodiments, there is no amplification of the axial stroke of one of the elements in order to actuate the corresponding contactor.